The present invention relates to a single crystal wafer of lithium tantalate or, more particularly, to a single crystal wafer of lithium tantalate provided with an orientation flat having at a specific crystallographic orientation.
As a material having a high electromechanical coupling coefficient and small temperature dependency of various parameters as the inherent characteristics, single crystals of lithium tantalate are widely used as a promising material for various kinds of SAW devices such as resonators in VCR instruments, PIF filters in televisions sets and the like as well as other devices useful in high frequency communication instruments.
As is known, single crystal boules of lithium tantalate are usually grown by the so-called Czochralski method from a melt of an oxide mixture having a composition of LiTaO.sub.3 by pulling up a boule on a seed in the direction of x-axis. The single crystal boule thus grown is first subjected to a poling treatment to align the ferroelectric domains into a single domain and then sliced perpendicularly to the x-axis into socalled x-cut wafers from which various devices are manufactured.
It is a usual practice that the x-cut wafer of a lithium tantalate single crystal is provided with an orientation flat, i.e. a flat lateral surface formed by grinding in a specified crystallographic orientation to facilitate subsequent works of fabrication into devices. In the x-cut single crystal wafers of lithium tantalate, the orientation flat is formed usually by grinding the lateral surface parallel to the 112.degree. Y direction located by the back-reflection Laue method or other suitable method in view of the easiness in distinguishing the direction of the propagation of ultrasonic surface acoustic waves in the preparation of devices. In other words, the orientation of the orientation flat is at the 112.degree. Y direction which should be the desired propagating direction of the ultrasonic surface acoustic waves.
A problem in forming the orientation flat in such an orientation is that, due to the absence of any crystalline lattice plane in this direction effective for the diffraction of X-rays, it is necessary that the single crystal is positioned at the correct direction by use of a holding jig of the crystal or by fine adjustment of the crystal direction by the back-reflection Laue method so that, even by setting aside the poor efficiency in the positioning works, the accuracy in the direction of the crystalline orientation is unavoidably low, consequently resulting in the poor performance or low quality of the devices prepared from the single crystal wafers or in the low yield of acceptable devices prepared therefrom.